Tunable coupling network for pushpull amplifiers



April 19, 1949- J A. REDMOND 2,467,778

TUNABLE GOUPLING NETWORK FOR PUSH-PULL AMPLIFIERS Filed Sept. 29, 1944 6' 6 nvpur CIRCUIT x, TBAVELLl/VG CAB/2046f WITNESSES: lNVENTOR Jo/mfi. Redmond.

I BY

Patented Apr. 19, 1949 TUNABLE COUPLING NETWORK FOR PUSH- PULL AMPLIFIERS John A. Redmond, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 29, 1944, Serial No. 556,415

4 Claims. 1

My invention relates to arrangements for coupling a radio frequency amplifier to an antenna in such a way that the arrangement may be tuned to various frequencies and, in particular, relates to arrangements of the type just described which have been found particularly advantageous in ultra high frequency service.

In systems in which radio frequency tubes supply currents to antennas, considerable difficulty has been encountered in finding arrangements for suitably coupling the tuned circuits of the tubes to the antenna in cases where it is desirable to tune the transmitter over a considerable range of frequencies. This difficulty has been particuiarly marked in the case of ultra and very high frequency transmitters, and also where the antenna is connected to the tubes through transmission lines of some length. One factor in causmg the difficulty is the fact that the reactance of the transmission system of the antenna varies considerably in the case of transmission lines of difierent length and also over the range of frequencies for which the transmitter frequency must be varied. A somewhat analogous difficulty has been encountered where the final stage of the transmitter embodies tubes arranged in pushpull connection and this feeds into a transmission line which has one conductor grounded.

While, of course, circuits for coupling pushpull transmitter tubes to antennas have been previously used in the art, they have shown in practice difficulties from the fact that they permit considerable reaction between the tuning of the antenna and the tuning of the tube circuits of the push-pull tubes; such as a limited range of tuning for the antenna system; the necessity for providing regulatable mutual inductance between components of the antenna and of the tube circuit; the requirement for an impracticably low degree of coupling between circuits in the case of ultra high frequency systems; the impossibility of determining antenna loading from the indications of the tube plate ammeters, and the like.

By employing a circuit in which the final tube stage of the transmitter employs a tank circuit which floats freely in potential relative to earth and in which the ungrounded side of the transmission line leading to the antenna is connected to a variable tap on the tank circuit inductance, I have avoided the use of separate primary and secondary windings in the couplings between the tube circuits and the antenna and have otherwise avoided the difficulties above enumerated.

One object of my invention is accordingly to provide a radio frequency transmitter with a tank circuit coupled to a transmitting antenna utilizing a minimum number of tuning elements.

Another object of my invention is to provide a transmitter coupled to an antenna which is capable of being tuned through an extremely wide range of frequencies.

Still another object of my invention is to provide an arrangement in which a transmitter tube is coupled to an antenna for ultra high frequency use in which the optimum tuning of the system can be determined from the maximum reading of an ammeter in the plate circuit of the tube in the final tube stage.

Still another object of my invention is to provide an arrangement in which a load circuit may be connected to a radio transmitter through a transmission line having one side grounded by the use of a minimum number of circuit elements.

A further object of my invention is to provide an arrangement in which a load circuit may be connected to a push-pull transmitter circuit by a circuit of minimum degree of complexity.

Other objects of my invention will become apparent upon reading the following description taken in connection with the drawings in which:

Figure 1 is a schematic diagram of a circuit connecting a pair of push-pull output tubes on a transmitter with an antenna through a transmission line having one side grounded, and

Fig. 2 is a detailed showing of the structure of a variable tuner which may be employed in carrying out the principles of my invention.

Referring in detail to Fig. 1, a pair of tubes i, 2 constitute the push-pull output stage of a radio transmitter having their control electrodes, 3, i respectively connected to the opposite terminals 5, 6 of an input circuit supplying them with suitable radio frequency power. The cathodes l, 8 of these tubes are connected together and then to ground through a resistor 9 and a capacitor I l. The anodes I3, [4 of the tubes I, 2 are connected respectively through choke coils l5, is to the positive terminal I! of a suitable direct current source of which the negative terminal is grounded. The anodes I3 and M are respectively connected through capacitors l8, Hi to a pair of variable taps 2|, 22 on an inductance comprising a pair of coils 23, 24. The variable taps 2|, 22 are respectively connected to opposite terminals of a capacitor 25. In many cases the distributed capacity of the circuit may be used instead of capacitor 25. A variable tap 29 on the inductances 23, 24 is connected to the ungrounded conductor 3| of a transmission line 32 leading to an antenna 33 or other suitable output load. The other conductor 34 of the transmission line 32 is connected to ground.

It will be noted that the inductors 23, 24 and capacitor 25 constitute a tank circuit which is freely floating in electrical potential relative to earth. This tank circuit may be tuned at will by altering the position of the taps 2i, 22 along the inductances 23, 24.

The above described circuit and antenna may conveniently be tuned in the following manner. The tap 29 is moved to a position halfway be.- tween the adjacent ends of the inductances 23, 24, under such a condition substantially no power is delivered to the antenna'33. The positions of the taps 21, 22 are then varied in opposite directions always maintaining them in symmetrical relation relative to the common terminal of the inductances 23, 24 until the tank circuit 23, 23, 25 is tuned to resonance with the incoming car: rier frequency. Resonance will be indicated by the fact that the ammeter 35 in the direct current cathode return circuit of the amplifier tubes shows a minimum reading. The tap 29 is then moved from the common terminal of the inductances 23, 28 for a short distance, and the positions of the taps 2i, 22 are again varied until resonance is indicated by a minimum reading on the ammeter 35. The position of the tap 29 is again moved further from the common. terminal of the ind-uctances 23, 24, and the movable taps 2!, 22 again adjusted until the ammeter 35 reads a minimum. This procedure is followed until the reading of the ammeter 35 shows that the power input to the tubes I, 2 suffices to deliver the desired power tothe antenna 33.

Turning to Fig. 2, a convenient tuning arrangement for applying the circuit of Fig. 1 particularly in the case of very high frequency transmitters comprises an inductor comprising two helical members 23, 24 which may be made up of a cylindrical rod or tube of copper of sufficient diameter to be self-supporting. The adjacent ends of the inductors 23, 2 2 are connected to each other by a leader M which is supported on a suitable insulator 425.

The central axis of the inductors 2'3, 24 is supported (in bearings not shown) by a shaft 43 having radially extending arms 54, 45, 4e and 41. The arms 44, 45 support a cylindrical shaft 48 on which a suitable trolley wheel 2! is supported in sliding engagement so that it can move along the axis of the shaft 4-8. Similar shaft M is supported on the arms 45, 41 and carries a trolley wheel 22 similar to the trolley wheel 2|. The trolley wheel 2! is preferably curved to fit the inner periphery of the helical winding 23 so that it makes running contact therewith similar to that between the trolley wheel and trolley wire in a street railway system. The same remarks apply to the trolley wheel 22 and the helical in.- ductor 24. The shaft 48 is connected to the capacitor IS in the Fig. 1 arrangement, and the shaft 5| is connected to the capacitor l9 therein. The Opposite ends of the helical inductors 23, 2d are respectively connected by horizontal leads 53 and 54 carried by suitable supporting insulators 55and 58.

A carriage til which may be of conductive material is arranged to support a cylindrical shaft 58 parallel to the axis of the shaft 33 and to move the shaft in such a parallel position circumferentially about the helical inductor 23. A trolley 29 which is arranged so that it can slide along the shaft 58 makes contact with the exterior peripheryot the inductor 23. By rotating the carriage '4 51 circumferentially about the inductor 23, the position of contact between the trolley 29 and the inductor 23 can obviously be displaced as much as desired from the end of the inductor 23 adjacent the strap 4|. A suitable lead 59 connects the carriage 5'! to the ungrounded conductor 3| of the transmission line 32..

The portion of the inductors 23, 24 which are included in circuit with the capacitor 25 can obviously be varied by rotating the shaft 43. The position of tap 29 along the inductor 23 can obviously be independently varied by rotating the carriage 51 about the inductor 23.

While I have complied with the patent statutes, I have described a particular embodiment of my invention, but the principles thereof are of broader application in ways which will be evident to those skilled in the art.

I claim as my invention:

1. In combination with a pair of push-pull connected output tubes each having an anode and cathode, a tank circuit including an inductor having a pair of variable taps, means for displacing said taps along said inductor always in symmetrical relation relative to the midpoint thereof, and means for connecting said variable taps to the anodes of said push-pull tubes, at third variable tap for said inductor, a load circuit, means for conne in s h d a ia le ta t one s de of said load circuit, and means fer connecting the other side of said load circuit to the cathodes of sa u h-Pu bes 2- I semi a on w th a s at esli putll we nected output tubes each having anode and cathode, a tank circuit including an inductor having a pair of variable taps, means for disp g said taps along said inductor always symmetrical relation relative to the midpoint thereof, and means for connecting said variable taps to the anodes of said pushepull tubes, a third variable tap for said inductor, a transmission line having one side grounded, means for connecting said third tap to the ungrounded side of said transmission line, and means for connecting the catln odes of said push-pull tubes to ground.

3. In combination with a pair of push-pull connected output tubes each having an anode and cathode, a tank circuit including an inductor having a pair of variable taps, means for displacing said taps along said inductor always in symmetrical relation relative to the midpoint thereof, and means comprising a pair ef eapacitors for con. necting said variable taps to the anodes of said push-pull tubes, a third variable tap for said inductor, a load circuit, means fer connecting said third variable tap to one side of said lead eircuit, and means f on cting th other s de o sai l d circui to h cathodes o sai rushu tubes. 7

4. In combination with a pair of pusnepull connected output be each ha in an anode and cathode, a tank c rcu in ludin a in uct r ha s a r of iab t s m ans er displacin said taps along said inductor always in symmetrical relation relative to the midpoint thereof, and means comprising a pair of capacitors for con.- necting said variable taps to the anodes of said push-pull tubes, a third variable tap for said inductor, a transmission line having one side grounded, means for connecting said third tap to the ne o d i ef said tr nsmis on line n means for on ec in the ath d of said push-pull tub es to ground,

JoHN A. REDMQ D- e erences on ollowin pa e) 5 REFERENCES CITED Number The following references are of record in the file of th1s patent. 2:178:221 UNITED STATES PATENTS 5 2,222,169

Number Name Date 1,647,259 Vadersen Nov. 1, 1927 1,854,008 Weir et a1. Apr. 12, 1932 gi if 1,880,198 Gebhard Oct. 4, 1932 1,894,498 Rudolph Jan. 17, 1933 10 Name Date Gebhard. Jan. 8, 1935 Leach Apr. 7, 1936 Blancha Oct. 31, 1939 Buschbeck et a1. Nov. 19, 1940 FOREIGN PATENTS- Country Date Great Britain Jan. 23, 1931 

